Safety trenching box

ABSTRACT

A safety trenching box is provided having a pair of earthretaining wall members which are tapered at one end thereof to form a pair of vertical cutting edges. The vertical cutting edges permit the box to be dragged through trench excavations having little or no clearance between the sides of the box and the sides of the trench, since the edges shear off projecting portions of the trench walls. The bottom edge of each wall member is also tapered to form a pair of horizontal cutting edges on the box and push plates are provided on the top edge of each wall member to permit the trenching box to be driven downwardly into the trench by externally applied forces. The outer surface of each of the wall members in contact with the sides of the trench is cambered to provide the box with increased structural strength under laterally applied load conditions. Horizontally disposed beams employed in the construction of each wall member are provided with inclined cover plate to form skid paths for earth and stone accidentally dropped into the trenching box, thereby preventing any dangerous accumulation of such materials on such beams and adding to the structural strength of the box. If desired, legs may be mounted on the bottom tapered edges of the wall members to facilitate the stacking of a number of such boxes for use in deep trench excavations or for storage purposes.

United States Patent [72] Inventor John R.Pavese 771 Rivervale Road, Rivervale, NJ. 07675 [21] Appl. No 885,252 [22] Filed Dec. 15, 1969 [45] Patented July 20,1971

[54] SAFETY TRENCHING BOX 2 Claims, 5 Drawing Figs.

[52] U.S.CI 61/41 [51] 1nt.Cl E21d 7/00 [50] Field ofSearch 6l/41,41 A, 72.1, 46, 72.3

[56] References Cited UNITED STATES PATENTS 694,398 3/1902 Maas 61/41 2,865,178 12/1958 Wicke 61/41 A 2,922,283 1/1960 Porter..... 61/41 A 3,362,168 1/1968 Dotlich .o 61/41 A 3,479,827 11/1969 Morrice 61/41 FOREIGN PATENTS 1,140,888 12/1962 Germany 61/41 Priman Examiner --Dennis1, Taylor Attorney-Robert A, Maikis ABSTRACT: A safety trenching box is provided having a pair of earth-retaining wall members which are tapered at one end thereof to form a pair of vertical cutting edges. The vertical cutting edges permit the box to be dragged through trench excavations having little or no clearance between the sides of the box and the sides of the trench, since the edges shear ofi projecting portions of the trench walls. The bottom edge of each wall member is also tapered to form a pair of horizontal cutting edges on the box and push plates are provided on the top edge of each wall member to permit the trenching box to be driven downwardly into the trench by externally applied forces. The outer surface of each of the wall members in contact with the sides of the trench is cambered to provide the box with increased structural strength under laterally applied load conditions, Horizontally disposed beams employed in the construction of each wall member are provided with inclined cover plate to form skid paths for earth and stone accidentally dropped into the trenching box, thereby preventing any dangerous accumulation of such materials on such beams and adding to the structural strength of the box. If desired, legs may 7 PATENTEB JUL20 I97! SHEET 1 BF 2 INVVI'YN'I (m. 7 JOHN R. PA VESE W 2 4 I ATTORNEY PATENTEO JUL20 |97| SHEET 2 0F 2 i INVl-IN'I ()Rv 5 JOHN R. PAVESE M M v A TTORNEY SAFETY TRENCI-IING BOX 1. Field of the Invention This invention relates generally to safety devices for use in the construction industry and more particularly to a safety trenching box which is especially suited for use in trench excavations and the like having a close-working clearance between the sides of the box and the sides of the trench.

2. Description of the Prior Art Safety trenching boxes or so-called safety drag boxes" are commonly employed in the construction industry during earth excavation work to prevent the walls of the excavation from collapsing and filling the excavated area. The problem presented by the collapse of the walls of an excavation is particularly acute in trench excavations of the type commonly made during the installation or repair of water'and sewer pipes, for example, where a somewhat deep butnarrow trench is dug to conform to the layout of the pipe system. When a back hoe or similar piece of mechanical digging equipment is employed, the walls or sides of the trench are usually substantially vertical and are sharply defined, thereby increasing the possibility that the sides of the trench may collapse to cause death or injury to workmen working at the bottom of the trench.

The known types of safety trenching boxes generally includes a pair of vertically disposed wall members which act to keep or retain the earth of the trench walls in positionfThe wall members are separated and laterally supported by at least two spreader members which are fabricated in varying widths to permit the trenching box to be used in trenches of various widths. The spreader members are bolted to the wall members of the box to create a trenching box of the proper width for a particular job. As the trench is dug, the box is repositioned for each section under construction by either dragging it through the trench or lifting it out and reinserting it at the next location. It is usually desirable to drag the box through the trench rather than lift the box in and out of the trench for repositioning, since the latter method requires expert handling of the lifting equipment and is time consuming .as well. 'With presently known safety drag boxes, however, the dragging operation becomes difficult when the side walls of the trench narrow down to a point where they exert a substantial frictional drag on the walls of the box. Minor projections and outcroppings on the trench walls may even make further movement impossible and the box must usually be removed from the excavation for repositioning. A somewhat similar problem may arise when the safety trenching box is being lowered into position'in a trench having a width too small to accommodate the box. In this situation, the box may become jammed after being lowered a distance into the trench and considerable time and efi'ort may be required to remove it.

Safety hazards may also exist with trenching boxes employing an open beam construction. In this type of construction, each of the wall members of the box is formed by a sheet metal member, usually steel, which is supported by a vertically disposed frame. The frame itself is formed by upper and lower horizontal beam members which are connected by vertically disposed columns to provide the needed structural strength. When loose earth or stones are accidentally dropped into the trenching box they may become lodged on the upper surface of the horizontally disposed beam members and may accumulate to a point where a slight movement of the box will send a heavy mass of such material crashing down on workers employed within the box.

SUMMARY OF THE INVENTION It is an object of this invention to provide a safety trenching box which is especially adapted for use in excavations, such as trenches and the like, wherein close working conditions are encountered.

It is a further object of this invention to provide a safety trenching box which is readily repositioned in the trenchbeing excavated by being dragged through the trench or by being lowered or driven vertically into the trench with a minimum of effort. 1... It is a still further object of this invention to provide a safety trenching box which is so constructed as to prevent a darngerous buildup or accumulation of earth and stones on the sides of the box under adverse working conditions.

It is another object of the v invention to' provide a safety trenching box which is so constructed as to facilitate the stacking of a plurality of boxes for use in deep trench excavations or for storage of the boxes.

It is an additional object of this invention to provide a safety trenching box of exceptionally strong construction which is capable of easily withstanding the large lateral forces exerted on the walls of the box by the sides of deep trench excavationsl- Briefly, the. safety trenching box of the invention comprises a pair of earth-retaining wall members each of which] is tapered atone end thereof to form a pair of vertically disposed cutting edges on the box, and a pair of spreader members which are connected between the wall members. for separating and laterally supporting the wall members against the lateral force exerted by the sides of a trench or other excavation. When the trenching box of the invention is dragged through a trench, the pair of vertically disposed cutting'e'dges function as side cutters to widen the trench by shearing off earth and stone projecting from the sides ofthe trench The bottom edge of each of the wall members is also tapered to form a' pair of horizontally disposed cutting edges on the bottom of the box and push plates are provided on the top, edge of each of the wall members to provide load bearing surfaces for the application of externally applied forces, "to thereby permit the trenching boxto be driven downwardly into a trench by the application of external forces to the push plates, so that the horizontal cutting edges also function as side cutters to widen the trench. The invention also contemplates the'addition of legs to the tapered portion of the bottom edge of each of the wall members, so that the box is adapted to be placed on top of another box in a stacked arrangement, to thereby permit multiple stacked boxes to be employed in trenc'lies'havinga: depth greater than the height of a single box and to facilitate storage of the boxes. I I

Each of the wall members of the safety trenching box of the invention may comprise a rigid frame formed by top and bottom beam'me'mbers and a plurality of column members interconnecting the beam members; A sheet member is mounted on the frame to cover it and to provide a relatively smooth outer surface for the wall member in contact with the sides of the trench. Cover plate means are disposed between each of the column members and are arranged to connect the inner surface of the sheet member with the upper surface of the bdtf tom beam member along the length of the bottom beam member, to provide an inclined skid path for earth and stone falling into the trenching box. The outer surface of each of the wall members adjacent the side of the trench ispreferably cambered to provide a convex curvature therefor in the horizontal pane of the box, to thereby increase the lateral load-carrying capacity of the box. g The nature of the invention and'other objects and additional advantages thereof will be more readily understood bythose skilled in the art after consideration of the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a perspective view of a safety trenching box constructed in accordance with the teachings of the present invention;

FIG. 2 is an elevational view of one of the wall rn ernbers of the safety trenching box shown in FIG; 1 of the drawings with the spreader members and the other wall member removed,

FIG. 3 is a fragmentary perspective view of the front portion safety trenching boxes constructed in accordance with the teachings of the invention showing the boxes in a stacked arrangement and indicating in dotted outline the addition of legs to the upper trenching box to facilitate the stacking of the boxes.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Referring now to FIG. 1 of the drawings, there is shown a safety trenching box constructed in accordance with the teachings of the present invention comprising a first wall member 10, a second wall member 11, a front spreader member 12, and a rear spreader member 13. As seen in FIGS. 2, 3 and 4 of the drawings, the wall member has a substantially rectangular, rigid frame 14 which is formed by a top beam member 15, a bottom beam member 16, and column members 17, 18, 19 and 20. The top beam member 15 is horizontally disposed and may conveniently comprise a structural steel channel. The bottom beam member 16 is also horizontally disposed and may conveniently comprise a structural steel I beam. The vertically disposed columns 17, 18, 19

and 20 are preferably structural steel I beams or wide flange sections. All of the columns and beam members forming the frame 14 may be welded together to form a rigid assembly of great structural strength.

A sheet member 21, which may be fabricated from steel sheet, is arranged to cover the frame 14 and is preferably secured thereto by means such as welding, to provide a smooth outer surface for the wall member 10. The provision of a smooth outer surface for each of the wall members is desirable to reduce the frictional drag on the trenching box when the box is being dragged through or lowered .into a trench having little or no clearance between the sides of the box and the sides of the trench. In order to increase the lateral load-carrying capacity of the box, i.e., the ability of the box to withstand forces exerted by the sides of the trench, the sheet member 21 is cambered outwardly to produce a convexouter surface for each of the wall members in contact with the sides of the trench when viewed in the horizontal plane of the box. The cambering of the outer surface of the sheet member 21 may conveniently be accomplished by bending or curving the top beam member 15 in the horizontal plane and similarly bending the bottom beam member 16, so that the entire frame 14 is cambered in the desired manner. The sheet member 21 may then be welded to the beam and column members forming the frame to thereby produce the convex outer surface for the wall member. Although the degrceof camber is not critical and is too small to be adequately shown in the drawings herein, it may be mentioned that a satisfactory camber for a wall member having a length of 20 feet would produce a bowing out of approximately one inch measured at the midpoint of the wall member between the ends thereof.

As may best be seen in FIG. 3 of the drawings, one edge 23 of the sheet member 21 is arranged to extend a distance beyond the front column member 17 which forms the front end of the frame 14. A front cover plate member 22 is then welded to the extended edge 23 of the sheet member and to the inner flange or surface of the front column member 17. If

desired, the top and bottom beam members may be tapered at.

the ends thereof adjacent the front cover plate 22 to completely enclose the front end of the wall member, and one or more triangular gusset plates 24 may be inserted between the front column 17, the sheet member 21 and the cover plate 22 to provide increased structural strength for the assembly. The foregoing arrangement provides a vertically disposed cutting edge along the extended edge 23 of the sheet member 21. Since thecutting edge lies within the outer surface of the sheet member 21, it functions as a side cutter to shear off projecting portions of the sides of the trench when the trenching box is dragged or pushed through the trench.

As seen in FIG. 3 of the drawings, the bottom edge 26 of sheet member 21 is arranged to extend below the bottom surface of the bottom beam member 16, so that a bottom cover plate 25 may be mounted between the bottom edge 26 of the sheet member and the inner flange or surface of the beam member 16, to thereby form a horizontally disposed cutting edge along the entire length of the wall member 10. Again, the

cover plate 25 may be secured to the flange of beam 16 and to the bottom edge of sheet member 21 by means such as welding and a plurality of gussets 27 may be inserted between beam 16, sheet member 21 and cover plate 25 at spaced intervals along the entire length of the bottom of the wall member to provide additional structural strength. If the abutting ends of the front cover plate 22 and the bottom cover plate 25 are tapered as shown along the line 28, and welded together, the front end and bottom of the wall member 10 are effectively enclosed to provide smooth surfaces adjacent the vertical and horizontal cutting edges of the box. In practice, the front cover plate 22 and bottom cover plate 25 may be fabricated from steel plate.

The invention also contemplates the use of push plates or pads, 29, 30, 31 and 32 which are respectively mounted in vertical alignment with columns 17, 18, 19 and 20 on the top surface of the beam member 15. The push plates may all be rectangular in shape, with the exception of push plate 29 which is tapered to fit the tapered cutting edge at the front end of the wall member 10. Push plates 29, 30, 31 and 32 may conveniently comprise steel plate and are preferably welded to the top surface of beam 15, so that each push plate functions as a load-bearing surface disposed above a column member. This arrangement permits the safety trenching box of the invention to be driven down into a trench or other excavation by external forces applied to the push plates, since the horizontally disposed cutting edge 26-on the bottom of each wall member functions as a side cutter to shear off projections from the side of the trench. The external forces, which may be supplied from construction equipment available at the site, such as a backhoe for example, are applied to the load-bearing surfaces provided by the push plates and are transmitted directly through column members 17, 18, 19 and 20 to the bottom cutting edge of the wall members. When the trench is of adequate width to receive the box or when the sides of the trench have only small projections to impede movement of the box, the box may, of course, be lowered into position without any externally applied forces.

In order to prevent falling earth and stone from accumulating on bottom beam member 16, top cover plates 33, 34 and 35 are disposed between adjacent column members and are connected by mean such as welding to the inner surface of sheet member 21 and the flange or upper surface of the bottom beam member. As seen in FIGS. 2, 3, and 4 of the drawings, the top cover plate 33 is disposed at an acute angle with respect to the inner surface of sheet member 21 and is connected to the flange portion of the beam member 16, so that the cover plate forms inclined skid path means between the columns 17 and 18 which serve to prevent earth, stones and other material from accumulating on top of the beam 16. Top cover plate 34 is similarly arranged between columns 18 and 19 and top cover plate 35 performs the same function between columns 19 and 20. Accordingly, any stones or other matter falling into the box will be guided to the bottom of the trench without building-up or accumulating on the beam 16. When earth or stone is permitted to accumulate on the sides of the trenching box, a slight movement of the box could cause the accumulated material to fall on workmen employed in the box and could produce serious injury or death. In deep exdistance to fall before hitting the bottom of the excavation. With the safety trenching box of the invention, however, any matter falling into the box will be guided immediately to the bottom of the trench even though a number of boxes are employed in a stacked arrangement for work in very deep trenches.

The top cover plates 33, 34 and 35 also function to increase the structural strength of the wall member and are preferably fabricated from steel plate. If desired, a plurality of gusset plates 36 may be disposed between the top cover plates and the inside of sheet member 21 at spaced intervals along the bottom beam member 16 to further increase the structural strength of the entire assembly. Additional cover plates 37, 38 and 39 may be employed between the bottom surface or flange of top beam member and the inner surface of the sheet member 21, as shown in FIGS. 2 and 4 of the. drawings, to further increase the structural strengthand stability of the unit. Since the lateral thrust exerted by the sides of the trench is applied to the outer surface of the sheet member 21 and is then transmitted to the frame 14, a plurality of vertically disposed stiffeners 40 are provided along the entire-length of the box on the inner surface of sheet member 21 between the cover plates 37, 38 and 39 and the cover plates 33, 34 and 35, to prevent buckling of the sheet member 21 and to thereby increase the structural strength of the unit. In practice, the stiffeners 40 may comprise steel angles which are welded to the inner surface ofthe sheet member 21; Finally, lift eyes 41 may be welded to the four inside corners of the wall member 10 formed by the beams 15 and 16 and the columns 17 and 20, to provide convenient means for lifting and moving the box. The four lift eyes may be fabricated of steel plate.

The wall member 11 is constructed in the same manner as wall member 10 and will not be described further herein, except to note that in the arrangement of its component parts it constitutes a mirror image of wall member 10, so that the vertical cutting edges are both disposed at one end of the trenching box and the horizontal cutting edges are both disposed on the bottom of the box. The spreader members 1 2 and 13 which separate and laterally support wall members 10 and 11 are usually made in several standard widths or sizes and are bolted to the wall members to permit the trenching box to be adjusted to handle trenches of different widths. As seen in FIG. 1 of the drawings, the front spreader member 1 2v may comprise a substantially rectangular frame or assembly 42 which is formed by horizontal beam members 43 and and vertical column members 45. The beam members 43 and 44 and column members 45 may be fabricated from structural steel and welded together to form a rigid assembly. The frame 42 is preferably welded to a sheet steel member 46 to enclose the end of the trenching box and to impart structural rigidity to the assembly. The ends of frames 42 are welded to a pair of end columns 47 which are connected to the end columns 17 of the wall members 10 and 11 by suitable connecting means.

When the connecting means comprise nuts and bolts, the front cover plate 22 of each wall member is formed with a plurality of apertures 48 adjacent bolt holes 49 in the front column member 17, to permit access to those nuts or bolts located within the closed end of each wall member formed by the column 17, cover plate 22 and sheet member 21. In practice, the apertures 48 may be formed by-castellating the edge of cover plate 22 which abuts the flange of column member 17. The apertures 48 may be omitted, of course, if the holes 49 are tapped to avoid the need for nuts, or if the nuts are welded to the interior surface of the flange portion of column 17. It is usually preferable, however, to employ separate nuts and bolts, since a bolt may shear in a tapped hole or the thread on a nut may become stripped at the construction site, thereby presenting a difficult and time-consuming repair job. The front spreader member 12, as thus far described, is of sufflciently rugged construction to permit the trenching box to be dragged through a trench by the bucket of a backhoe gripping the spreader member or to permit the box to be hammered into position in a trench by blows applied to the spreader 5 components at the line contact of each wall member with the member itself. When this type of operation is contemplated, however, the bolts employed in connecting the spreader member to the wall members should have a suitably high shear strength. For reasons which will be described hereinafter, the front spreader member 12" may be made sothat its. bottom width is slightly greater than its top width, thereby imparting a gradual taper to the box over theheight of the box.

The rear spreader member 13 may be fabricated in exactly the same manner as the front spreader member 12 and will not be described further herein. It may be noted, however, that the front spreader member is'customarily made somewhat wider than the rear spreader member, to provide'a gradual taper for the trenching box over its entire length. This arrangement permits the box to be dragged through a trench or other excavation with a min'imurii'of frictional drag. Under these conditions, it is believed apparent that the vertical cutting edges 23 formed at the front end of the box will serve as side cutters to widen the trench to the required width as-thc' box is dragged or pushed through the trench. 1f the trench is widened sufficiently to.- permit passage of the front end of the box, the tapered configuration of the box will insure that the rest of the box will pass through without binding.

Referring now to FIG. 5 of the. drawings, it is seen that two; or more of the safety trenchingboxesrof the invention may be employed in a stacked arrangement. When the boxes are stacked vertically, it is possible toutilize a numberfof such boxes to create a safe enclosure for trenches and: other excavations wherein the depth of the trench orexcavation is substantially greater than the height of a single trenchingbox. As

- seen in FIG. 5, the safety trenching box described in. F168. 1

through 4 ofthe drawings isplaced on top of an identical safety trenching box which has a wall member 101 anda wall member 102. By suitably proportioning the front and rear,

spreader members as previously described, the trenching boxmay be given a vertical taper so that the-distance between the bottom cutting edges is greater than the distance between the tops of the wall members. When this is done, the bottom cutting edges of-one box will straddle" the top of the box un.-. demeath it, as'illustrated -to-provide lateral stability for the stacked boxes. The vertical taper also functions to, prevent binding when the-box is loweredinto a trench having asmall clearance between the walls of the box and the sides, of the trench. f

When two boxes are stacked as shown in FIG. 5, it isseen that'the tapered bottom of each of the wall members-10 and 11 fonned by the cover plate 25 of the upper box provides only a line contact with the corresponding surface of the wall members 101 and 102 of the lower box 100. Accordingly, the weight of the top box and any force applied to the top boxis transmitted to the bottom box over the very small surface area provided by the line contact. Furthermore, since the bottom of wall members 10 and 11 is tapered, any vertical force applied to the upper box will be split into vertical and horizontal wall member of the box below, thereby decreasing the force applied to the cutting edges 103 (in the bottom box. Although the boxes may bestacked in this manner by securing" the respective lift eyes 41 of the boxes together by chains or cables (not shown), the invention also provides that support legs 52 (shown in dotted outline) may be mounted on the, bottom cover plate 25 of each of the wall members to provide flat, load-bearing surfaces of larger area between the two boxes. The legs 52, which may comprise truncated. steel 1 beams, are preferably welded to the bottom cover plates 25 in vertical alignment with the columns l7, l8, l9 and 20 of each wall member, so that they bear directly on the push plates mounted on top of the wall members 101 and. 102 of the lower trenching box 100. By virtue of this arrangement, if the stacked trenching boxes are hammered or driven downwardly into a trench by the application offorces to the push plates 29, 30, 31, and 32 of the top box, the'fvertical forces so applied will be transmitted through the columns. 17, 18, 19 and 20 and. the legs 52 of the upper box directly to the corresponding push plates and column members of the lower box 100, to thereby provide maximum driving force at the bottom cutting edges [03 of the bottom box.

It is believed apparent that many changes could be made in the construction and described uses of the foregoing safety trenching box and many seemingly different embodiments of the invention could be constructed without departing from the scope thereof. For example it is believed obvious that the particular structural configuration employed for the frame portion of each of the wall members of the trenching box could be varied in design and still provide the required structural rigidity and strength. Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What I claim is:

l. A safety trenching box adapted to be disposed in an excavated trench or the like to prevent the sides of the trench from collapsing comprising a pair of earth-retaining wall members, each of said wall members having a rigid frame formed by horizontally disposed top and bottom beam members and a plurality of vertical column members interconnecting said beam members, a sheet member mounted on and covering said frame to provide a smooth outer surface for the wall member in contact with the side of the trench, said sheet member having one edge thereof extending beyond the edge of said frame at one end of the wall member and the bottom edge thereof extending below the bottom beam member along the entire length of the wall member, first cover plate means connected between said one extended edge of the sheet member and the inner surface of the column member adjacentThereto to form a vertically ex tending cutting edge at said one end of the wall member, second cover plate means connected between said bottom extended edge of the sheet member and the inner surface of the bottom beam member to form a horizontally disposed cutting edge along the bottom of the wall member, and third cover plate means disposed between each of said column members, said third cover plate means connecting the inner surface of the sheet member and the upper surface of the bottom beam member along the length thereof to provide an inclined skid path on each wall member for earth and stone falling into the box; and

a pair of spreader members removably connected between the column members at the ends of the frames of said wall members for separating and laterally supporting said wall members against the lateral force exerted by the sides of the trench.

2. A safety trenching box as claimed in claim I, wherein push plates are mounted on the top beam member of each of said wall members in vertical alignment with the column members thereof, and

at least two legs are mounted on the second cover plate means of each wall member in vertical alignment with said push plates, so that said push plates are adapted to function as'load-bearing surfaces for the application of vertically applied forces to permit the box to be driven downwardly into the trench and also function as load bearing surfaces for the legs of another box when two or more boxes are employed in a stacked arrangement. 

1. A safety trenching box adapted to be disposed in an excavated trench or the like to prevent the sides of the trench from collapsing comprising a pair of earth-retaining wall members, each of said wall members having a rigid frame formed by horizontally diSposed top and bottom beam members and a plurality of vertical column members interconnecting said beam members, a sheet member mounted on and covering said frame to provide a smooth outer surface for the wall member in contact with the side of the trench, said sheet member having one edge thereof extending beyond the edge of said frame at one end of the wall member and the bottom edge thereof extending below the bottom beam member along the entire length of the wall member, first cover plate means connected between said one extended edge of the sheet member and the inner surface of the column member adjacent thereto to form a vertically extending cutting edge at said one end of the wall member, second cover plate means connected between said bottom extended edge of the sheet member and the inner surface of the bottom beam member to form a horizontally disposed cutting edge along the bottom of the wall member, and third cover plate means disposed between each of said column members, said third cover plate means connecting the inner surface of the sheet member and the upper surface of the bottom beam member along the length thereof to provide an inclined skid path on each wall member for earth and stone falling into the box; and a pair of spreader members removably connected between the column members at the ends of the frames of said wall members for separating and laterally supporting said wall members against the lateral force exerted by the sides of the trench.
 2. A safety trenching box as claimed in claim 1, wherein push plates are mounted on the top beam member of each of said wall members in vertical alignment with the column members thereof, and at least two legs are mounted on the second cover plate means of each wall member in vertical alignment with said push plates, so that said push plates are adapted to function as load-bearing surfaces for the application of vertically applied forces to permit the box to be driven downwardly into the trench and also function as load bearing surfaces for the legs of another box when two or more boxes are employed in a stacked arrangement. 